ldbl-128ibm-compat: Fix selection of GNU and ISO C99 scanf

Since commit

commit 03992356
Author: Zack Weinberg <zackw@panix.com>
Date:   Sat Feb 10 11:58:35 2018 -0500

    Use C99-compliant scanf under _GNU_SOURCE with modern compilers.

the selection of the GNU versions of scanf functions requires both
_GNU_SOURCE and -std=c89.  This patch changes the tests in
ldbl-128ibm-compat so that they actually test the GNU versions (without
this change, the redirection to the ISO C99 version always happens, so
GNU versions of the new implementation (e.g. __scanfieee128) were left
untested).

Tested for powerpc64le.

Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>